In a conventional wireless communication system, high-quality and a large amount of multimedia data should be transmitted through a limited frequency band. This calls for development of methods for transmitting a large amount of date through the limited frequency. The methods can be realized in the form of a Multiple Input Multiple Output (MIMO) system.
The MIMO system forms a plurality of independent fading channels by using multiple antennas in transmission/reception blocks and it can improve data transmission rate by transmitting different signals in each transmission antenna. Therefore, a large amount of data can be transmitted without expending frequency bandwidth.
However, the MIMO system has an inter symbol interference (ISI) occurring in high-speed data transmission, and weak frequency selective fading. In order to overcome these disadvantages, the MIMO system is applied to an Orthogonal Frequency Division Multiplexing (OFDM) method. The OFDM method is a multiplexing method which is most suitable for current high-speed data transmission. In the OFDM method, one data stream is transmitted through a sub-carrier frequency having a low data transmission rate. A channel environment for wireless communication has multiple paths due to obstacles, e.g., buildings. In a wireless channel having the multiple paths, a delay spread occurs due to the presence of multiple paths, and the ISI occurs when time of delay spread is greater than transmission time of the subsequent symbol. When frequency selective fading occurs in a frequency domain and a single-carrier is used, an equalizer is used to eliminate the ISI. The higher data transmission rate is, the more complex the equalizer is.
As a result, when the MIMO system and the OFDM system are integrated, advantages of the MIMO system are used and disadvantages of the MIMO system can be offset by the OFDM system. A general MIMO system has N transmission antennas and N receiving antennas. Thus, a MIMO-OFDM system is formed by integrating the MIMO system and the OFDM method.
FIGS. 1 and 2 are block diagrams illustrating a conventional Multiple Input Multiple Output-Orthogonal Frequency Division Multiplexing (MIMO-OFDM) system.
FIG. 1 is a transmission block of the MIMO-OFDM system. The transmission block includes a serial-to-parallel (S/P) converter 101, an encoder 102, a quadarure amplitude multiplexing (QAM) mapper 103, an inverse fast Fourier transformer (IFFT) 104, a cyclic prefix (CP) adder 105, and a digital-to-analog conversion and radio frequency processing unit (D/A and RF unit) 106.
The S/P converter 101 divides transmission data into a plurality of data streams to be transmitted through transmission antennas before encoding. Each encoder 102 encodes each data stream. The QAM mapper 103 modulates the encoded data stream based on a modulating method, e.g., binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16 quadrature amplitude multiplexing (16QAM) and 64QAM, and generates a modulated symbol.
The IFFT 104 transforms the modulated symbol into a time domain symbol. The CP adder 105 adds a CP to a front of the time domain symbol. In the D/A and RF unit 106, the D/A converter converts the CP-added symbol into an analog signal, and the analog signal is transmitted through the RF unit.
FIG. 2 is a reception block of the MIMO-OFDM system. The reception block includes a analog-to-digital conversion and radio frequency processing unit (A/D and RF unit) 107, CP eliminator 108, fast Fourier transformer (FFT) 109, a MIMO receiver 110, decoder 111, and a parallel-to-serial (P/S) converter 112.
In the A/D and RF unit 107, an analog signal is received through the RF unit, and the A/D converter converts the received analog signal into a digital signal. The CP eliminator 108 eliminates a CP from the digital signal. The FFT 109 performs FFT to each of parallel digital signals.
The MIMO receiver 110 estimates a symbol of transmission data, and calculates a log likely-hood ratio (LLR) based on the estimated symbol. Each decoder 111 decodes each of data streams to estimate the transmission data. The P/S converter 112 serializes the parallel data streams into a serial data stream.
In examples of the MIMO receiver, there are a decision feedback equalizer (DFE), a zero forcing (ZF), a minimum mean square error estimation (MMSE) and a bell labs layered space-time (BLAST). Complexities of the above receivers are smaller than complexity of a maximum likelihood detection (MLD), but performances of the above receivers are not better than that of the MLD.